An unresolved and controversial question in human nutrition is whether infants have a specific nutritional need for docosahexaenoic acid (DHA, 22:6w3), a long-chain omega-3 fatty acid which is a major structural component of membranes in the retina and brain. Current US infant formulas contain linolenic acid, a dietary precursor of DHA, but do not directly provide DHA as does human milk. Our previous studies of rhesus monkeys demonstrated that diets low in omega-3 fatty acids during gestation and infancy led to reduced brain and retina DHA levels, deficits in retinal function, slower visual acuity development and changes in visual attention. These findings led to increases in the amount of linolenic acid in many infant formulas, and prompted studies of premature human infants which found similar effects of omega-3 fatty acids on all of these measures. Our current work is evaluating the influence of different amounts and forms of dietary omega-3 fatty acids on visual, cognitive and behavioral development. We are comparing three groups of rhesus monkeys fed a diet low in linolenic acid (18:3w3), a diet high in linolenic acid, or a diet containing DHA and simulating the fatty acid composition of primate milk. During the last year we completed all functional testing through 18 months of age, including behavioral and evoked potential measures of visual acuity and contrast sensitivity, electroretinographic studies of retinal function, and behavioral tests of visual attention, recognition memory, short-term working memory and response inhibition. Effects of low levels of linolenic acid were found on visual tests and on measures of visual attention, but supplementation with DHA showed no benefit compared with diets high in linolenic acid. In a new series of studies of immune system function, animals fed DHA showed reduced lymphocyte response to mitogen stimulation, suggesting altered immune function.